1. Field of the Invention
The present invention relates to a semiconductor device. More specifically, it relates to a semiconductor device including a thin-film transistor (TFT) prepared from crystallized silicon.
2. Description of the Background Art
In order to prepare a thin-film transistor from polycrystalline silicon, a step of crystallizing silicon is generally carried out. In this crystallization step, a treatment referred to as laser annealing is performed by applying a laser beam emitted from an excimer laser such as an Xe—Cl laser to an amorphous silicon film for melting the amorphous silicon film with heat resulting from this lasing and crystallizing the silicon in subsequent cooling. A polycrystalline silicon film can be obtained through this treatment. When the polycrystalline silicon film is prepared by this method, a substrate itself is hardly exposed to heat and hence a material having a low heat-resistant temperature can be employed for the substrate. Thus, a thin-film transistor can be formed on a glass substrate having a low heat-resistant temperature.
However, the laser beam emitted from the excimer laser such as an Xe—Cl laser and applied to the amorphous silicon film reaches only a portion of the silicon layer close to the surface thereof, and hence a layer having a large crystal grain size is formed only around the surface of the silicon layer. In relation to laser annealing, therefore, proposed is application of a YAG laser beam in place of the excimer laser beam.
According to a technique disclosed in Japanese Patent Laying-Open No. 2002-367904, a polycrystalline film formed by solid phase growth is extremely thinly left on a lower portion of a semiconductor film while the remaining region is melted for growing crystals from the left polycrystalline film formed by solid phase growth, as described in section 0033 with reference to FIGS. 1 and 4. In an embodiment of this technique, a heat treatment is performed on an amorphous semiconductor film for crystallizing the amorphous film in a solid phase (section 0059) and applying the second harmonic of an Nd:YAG laser beam to the intrinsic polycrystalline silicon film obtained by solid phase growth for melting/crystallizing the same (section 0060). The aforementioned gazette describes that about 80% of the semiconductor film is melted.
According to a technique disclosed in Japanese Patent Laying-Open No. 2000-269133, the second harmonic of an Nd:YAG laser beam is applied to an intrinsic amorphous silicon film for melting/recrystallizing the same (section 0023). This gazette describes that about 92% of the semiconductor film is melted.
A transistor formed on a silicon substrate has such a property that portions of crystal defects easily trap unnecessary impurities causing deterioration of transistor characteristics. Gettering can be performed through this property. In the transistor formed on the silicon substrate, a gettering site is constituted by depositing polycrystalline silicon on the back surface of the silicon substrate or a forming portion having a large number of crystal defects on the back surface of the silicon substrate by sandblasting or the like.
In a thin-film transistor formed on a glass substrate, on the other hand, the thickness of a silicon layer is so small that it is theoretically possible but inefficient to intentionally deposit a polycrystalline silicon film on the back surface of the silicon layer in consideration of a step necessary for this working. Further, it is impossible to perform a treatment such as sandblasting on the back surface of the silicon layer. In the thin-film transistor formed on a glass substrate, therefore, a gettering site must be constituted by another method.